It is hitherto known that spironaphthoxadine derivatives show photochromism. As a result, these compounds are now being employed in sunglasses, optical filters, curtains, etc. by dissolving or dispersing them in high polymeric binders.
U.S. Pat. No. 3,578,602 discloses 1,3,3-trimethylspiro[indoline-2,3'-(3H)-naphtho(2,1-b)-1,4-oxadine] and proposes a photochromic material prepared by coating a solution of this compound in an appropriate high polymeric binder on a transparent support and drying. However, this material has not yet been put in practical use since the change in the optical density at the maximum absorption wavelength in the state activated by light irradiation (hereinafter referred to as "color developability") is not satisfactory.
In order to overcome the above-described problem, an attempt was made in U.S. Pat. No. 4,215,010 to enhance color developability by introducing a methoxy group, an ethoxy group or a halogen atom into the 8'- or 9'-position of the aforesaid spironaphthoxadine compound, but such a proposal is still unsatisfactory from the standpoint of practical use.